The emission regulations coming into force in the next few years will, in particular, also lead to a tightening of exhaust gas limit values for motor vehicles equipped with diesel engines. From the present-day point of view, a consequence of this is that an after-treatment of the exhaust gas is absolutely necessary for removing nitrogen oxides.
U.S. Pat. No. 6,890,501 B2 describes the purification of offgases from industry, for example in the production of nitric acid. Here, use is made of catalyst materials in which ion-exchanged zeolites or a Fe2O3/beta-zeolite is proposed as support material which serves to join the active component to the substrate body. In the Fe2O3/beta-zeolite material, the iron oxide is applied to the support material by means of excess impregnation, which leads to a number of disadvantages. Firstly, the actual final content of iron oxide on the support material can in this way be determined only with difficulty, as a result of which this ratio differs from one synthesis to the next and quality differences are obtained as a result. Secondly, sometimes complicated filtration and/or purification processes are necessary after the synthesis in order to obtain the desired material.
EP 0 393 905 B1 describes an SCR (Selective Catalytic Reduction, i.e. the use of NH3 as reducing agent) catalyst which has an iron-containing beta-zeolite as support material and active component. The joining of the iron to the zeolite is carried out by means of ion exchange, so that the iron is present in ion-exchanged form on the zeolite. However, various experiments have shown that such catalysts in which the active component is present in ion-exchanged form have comparatively poor short- and long-term activities in the reduction of NOx-containing offgases.
US 2005/0031514 A1 describes the application of diesel particle filters which are coated with an SCR catalyst. Here too, the iron is present in ion-exchanged form, which leads to the abovementioned disadvantages.
EP 1 475 149 A1 discloses a catalyst for the reduction of NO to N2 by means of hydrogen under O2-rich conditions. This known catalyst is based on platinum which is distributed in an amount of from 0.1 to 2 per cent by weight on a support material comprising magnesium oxide or cerium oxide or a precursor thereof. Although quite good results are obtained in the reduction of NOx using this catalyst, this catalyst, too, could reach its limits in the case of future pollutant limit values. In addition, an in-principle problem with the presence of hydrogen over platinum-containing catalysts is that NO is also converted into the undesirable greenhouse gas N2O, also known as laughing gas.
A process for removing nitrogen oxides from an offgas stream is described in EP 0 666 099 B1. The catalyst used here adsorbs the nitrogen oxides present in the offgas, after which a gas having a particular content of a reducing substance is supplied to the catalyst at prescribed time intervals and for particular periods of time. However, such storage catalysts in which basic components such as lithium oxide, potassium oxide, sodium oxide, barium oxide or similar oxides are used require relatively complicated control and usually have a high regeneration requirement.
In the case of these NOx storage catalysts, the NO which is mainly emitted is oxidized over a catalyst comprising platinum to NO2 which is subsequently adsorbed on specific storage media, for example BaCO3. When the storage capacity of this catalyst is exhausted, a engine-induced regeneration of the catalyst in which the nitrogen oxides introduced are converted into nitrogen is commenced.
A further disadvantage of the known NOx storage catalysts is the risk of poisoning of the NOx sorbents by the sulphur oxides SO2 and SO3 present in the offgas. To overcome these problems, complicated engine management strategies are usually necessary.
EP 0 960 649 B1 discloses an offgas purification catalyst in which the materials used comprise cerium oxide and/or zirconium dioxide mixed oxides which serve to remove saturated hydrocarbons from the offgas. Ammonia is used as reducing agent for the nitrogen oxides present in the offgas.
However, the active component V2O5 frequently used in such SCR catalysts is associated with toxicological concerns and can also melt or sublime at very high offgas temperatures (>650° C.).
A further disadvantage of the known solutions for the removal of NOx from O2-rich exhaust gases is in most cases that the nitrogen oxides are reacted effectively only above 200° C. Since the temperature of the exhaust gases is steadily being reduced as a result of the continual optimization of the efficiency of internal combustion engines, the known solutions are associated with a great problem in terms of their effectiveness. For example, in the case of modern internal combustion engines operating according to the diesel principle for passenger cars, the exhaust gas temperature in the relevant certification cycle is below 150° C. for about 60% of the time and below 200° C. for about 75% of the time.